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Underground coal mining practice often requires that some coal be lift in place at the seam boundaries after mining, for purposes of ground control, quality control, machine guidance, of good operating practice. Many methods for measuring the thickness of boundary coal have been developed, but only the natural gamma method has been used in routine operations. Use of this method is constrained by the level and consistency of the natural gamma background in the bordering strata, the accuracy of the gamma detection instrument, and the elevation profile of the seam boundary being detected. Computer simulations for three commercially available gamma detection instruments, designed for mine use, showed that measurement of boundary coal thickness by the natural gamma method provides satisfactory input for horizon control systems when the gamma detector is large, when the sensor is mounted on the mining machine, and when the elevation profile of the seam boundary does not change rapidly in the direction of advance. Installation of a natural gamma thickness sensor on a continuous miner in the Pittsburgh seam resulted in improved control of roof coal thickness, and in a significant decrease in auxiliary bolting required when the miner cut into the immediate roof. The average roof coal thickness measured after mining with input of the sensor was 175 mm; the average thickness predicted by the simulation was 149 mm.